An ultrasonic diagnostic apparatus is a diagnostic imaging apparatus that provides images inside of a subject on the basis of reflected ultrasonic waves by transmitting ultrasonic waves from an ultrasonic probe to a subject, and receiving reflected waves from regions of discordant acoustic impedance by the ultrasonic probe.
In a known ultrasound imaging device, a plurality of acoustic matching layers having thicknesses of less than a quarter of ultrasonic wave length are layered between a piezoelectric vibrator and an acoustic lens. This technique is intended to match the acoustic impedance of a piezoelectric vibrator to the acoustic impedance of the acoustic lens. Many matching layers result in acoustic matching between the piezoelectric vibrator and the acoustic lens. This technique thus contributes to a wider frequency bandwidth of ultrasonic waves transmitted and received and higher sensitivity for detecting ultrasonic waves.
At a same time, there exists a technique for making matching layers having slope characteristics of acoustic impedance. By this technique, a discontinuous part in the impedance characteristic of a matching layer disappears by matching the acoustic impedance of a piezoelectric vibrator to that of an acoustic lens without a break. This technique improves propagation efficiency of ultrasonic waves.
One concrete method of forming the matching layer is to evaporate at least two materials, while gradually changing the ratio of the two materials. (For example, see JP07-390A.) As another concrete method, a matching layer formed by arranging cone state materials and plastic filled between the cone state materials. This matching layer also has slope characteristics of acoustic impedance. (For example, see JP11-89835A.)
By virtue of this slope matching technique, because of nonexistence of a impedance discontinuity, it is expected that a reduction of reflecting loss, resulting in improved transmitting and receiving efficiency and wider bandwidth of ultrasonic wave, can be realized in comparison with the case of using two or three matching layers.
However, the slope matching layers of the prior art also have discontinuous faces. At a boundary face between an acoustic matching layer and an acoustic lens and a boundary face between an acoustic matching layer and a piezoelectric vibrator, the rate of change of acoustic impedance is discontinuous. In the following discussion, the discontinuous of rate of change is explained with reference to FIG. 14.
As shown in FIG. 14(a), acoustic impedance changes continuously from a piezoelectric vibrator to an acoustic lens. However, the rate of change of acoustic impedance is discontinuous at a boundary interface between an acoustic matching layer and an acoustic lens and a boundary face between an acoustic matching layer and a piezoelectric vibrator.
In consequence, this discontinuous interface causes generation of reflection from the boundary, which leads to a loss of ultrasonic wave and deters image diagnostics.